Many attempts have heretofore been conducted to compound a polyolefin resin with a filler, melt-mold the composition into a film or sheet, and monoaxially or biaxially stretch the resulting film or sheet to thereby produce a porous film.
However, the thus-produced monoaxially stretched films have problems with respect to anisotropy of physical properties thereof, particularly tearability in longitudinal (or stretched) direction and surface strength, and the biaxially stretched films have problems with respect to surface strength and stretchability, though they have no problems with anisotropy of physical properties thereof. Further, both films generally tend to possess high rigidity, which is not desirable in some uses.
One approach for removing anisotropy of physical properties of the film and improving surface strength is to conduct stretching at a stretching ratio as low as possible to render the film porous. However, no satisfactory results have so far been obtained.
In order to impart pliability to film, it is considered to add a low-melting polymer, a rubbery material, a plasticizer, a surfactant, and the like. However, there have so far been found no films having satisfactorily balanced physical properties such as porosity, stretchability, surface strength, etc.
As a result of intensive investigations to provide a porous film or sheet which removes the above-described defects with the conventional porous film or sheet, which, when monoaxially stretched, shows well balanced physical properties and possesses a strong surface strength and a high pliability, and which, when biaxially stretched, shows excellent surface strength, excellent stretchability, and a high pliability, the inventors have achieved the present invention.